A known step-down circuit of this type, described in commonly owned Italian Pat. No. 926,652 issued Aug. 17, 1972, comprises essentially a chopping transistor in series with a conductor connected to an ungrounded supply terminal and a smoothing filter downstream of that transistor. An ancillary direct-current source, in series with an intermittently closed electronic control switch, is inserted between the transistor base and the other supply terminal, i.e. ground. Periodic closure of the control switch briefly saturates the transistor and charges a shunt capacitor forming part of the downstream filter. The load voltage, developed across that capacitor, is therefore a fraction of the supply voltage determined by the open/closed ratio of the control switch.
Such a control circuit operates generally satisfactorily, especially with relatively low supply voltages of not more than about 100 V. With substantially higher voltages, however, the power dissipated by the circuit branch including the ancillary d-c source becomes significant.